1. Field of the Invention
The present invention relates to objective lens actuators, and more particularly to an objective lens actuator installed in the optical pickup used in the optical disk recording and/or reproducing device (hereinafter referred to as an optical disk device,) such as the minidisk (referred to as MD) player, the compact disk (referred to as CD) player, the digital video disk (referred to as DVD) player, etc., for displacing the objective lens to adjust the position of irradiation of light beam.
2. Description of the Background Art
As is well known, optical disk devices optically record and reproduce information to and from a disk-like information recording medium (hereinafter referred to as an optical disk) such as MD, CD, DVD, etc. while driving the objective lens along the two axes in the Z axis direction corresponding to the vertical optical axis direction as seen from the side of the optical disk (hereinafter also referred to as a focus direction) and in the X axis direction corresponding to the parallel radial direction as seen from the side of the optical disk (hereinafter also referred to as a tracking direction) to correct the focus error caused by up-and-down movement due to a warp of the optical disk and the tracking error caused by its eccentricity.
FIG. 22 is a diagram roughly showing the structure of an optical pickup installed in the optical disk device. The optical pickup is now be described briefly.
In FIG. 22, the light beam emitted from a semiconductor laser 111 is transmitted through a beam splitter 222 to enter an objective lens 1 provided in a lens holder 2. The objective lens 1 collects the laser light to form a small beam spot of about 1 .mu.m on the recording surface of an optical disk E. An objective lens actuator 555 is provided, being related to the lens holder 2. This objective lens actuator 555, which is formed of an electromagnetic circuit, is capable of driving the objective lens 1 in the focus direction and in the tracking direction. Provision of such an objective lens actuator 555 enables follow-up control of the objective lens 1 to the recording track with a precision of submicrons against movement of the surface or eccentricity of the optical disk E.
The light beam reflected at the recording surface of the optical disk E returns through the objective lens 1 and is reflected at the beam splitter 222 in the normal direction. Then the PIN photodiode 666 detects the intensity of the beam to enable reading of the information pits of the optical disk E and detection of the focus error signal and the tracking error signal.
Recently, optical disk recording/reproducing devices with higher density are being developed. To increase the resolution for high-density recording/reproducing, objective lenses with larger numerical aperture (hereinafter referred to as NA) are used. When the optical axis of the beam is inclined with respect to the recording/reproducing surface of the optical disk, however, the degree of coma increases in proportion to the third power of the NA. Therefore, with an objective lens having a large NA, if the optical axis is inclined while the objective lens is being moved in the focus direction or in the tracking direction in the objective lens actuator, it will exert an ill effect on the recording/reproducing of signal on the optical disk.
Conventionally, an objective lens actuator which solves the problem mentioned above is disclosed in Japanese Patent Laying-Open No.7-240031. The conventional objective lens actuator will now be described referring to FIG. 23 to FIG. 26.
FIG. 23 is a perspective view showing the structure of the conventional objective lens actuator. FIG. 24 is a cross-sectional view showing a main part of the objective lens actuator moving the objective lens in the focus direction (Z axis direction) and the tracking direction (X axis direction). FIG. 25 is a side view of the main part of the objective lens actuator moving the objective lens in the focus direction and the tracking direction. FIG. 26 is a top view of the main part of the electromagnetic circuit in the objective lens actuator in the case where a positional error occurs in the X axis direction.
In FIG. 23 to FIG. 26, the conventional objective lens actuator includes a lens holder 2, a focus coil 3, tracking coils 4, permanent magnets 5, a yoke base 6, back yokes 6a, opposing yokes 6b, elastic supporting members 7, a supporting member fixing portion 8, a supporting member fixing substrate 9, and printed boards 10.
The lens holder 2, which is formed of a resin molded article, holds the objective lens 1 fixed by adhesion or the like. The elastic supporting members 7, which are formed of a metal line having spring property, hold the lens holder 2, with their respective ends on one side solder-fixed to the printed boards 10. The back yokes 6a and the opposing yokes 6b form a magnetic circuit with the permanent magnets 5. The ends of the opposing yokes 6b on the side closer to the optical disk E are disposed closer to the optical disk E than the ends of the permanent magnets 5 on the side closer to the optical disk E. This forms a flow of magnetic flux in the focus direction in the part close to the optical disk E in the gap between the permanent magnets 5 and the opposing yokes 6b. The focus coil 3 and the tracking coils 4 are wound around the sides of the lens holder 2. The supporting member fixing portion 8 fixes the supporting member fixing substrate 9. The other ends of the elastic supporting members 7 are solder-fixed to the supporting member fixing substrate 9. The yoke base 6 is fixed on an optical substrate (not shown) supporting the semiconductor laser 111, the beam splitter 222, and the photodetector 666 in FIG. 22.
Next, the operation of driving the objective lens 1 along the two axes in the focus direction and the tracking direction to correct a focus error caused by the up-and-down movement due to a warp of the optical disk E and a tracking error cased by eccentricity or the like will be described.
The lens holder 2 to which the objective lens 1 is attached is supported in such a way that it can be moved in the focus direction and the tracking direction by the four elastic supporting members 7 disposed in parallel to each other (each of which has its one end fixed to the lens holder 2 through the printed board 10 and the other end fixed to the supporting member fixing substrate 9.) The supporting member fixing substrate 9 is fixed to the supporting member fixing portion 8 fixed to the yoke base 6.
A driving force in the focus direction is generated by the electromagnetic driving circuit in which the focus coil 3 is disposed in the gap in the magnetic circuit composed of the permanent magnets 5, the back yokes 6a and the opposing yokes 6b attached to the yoke base 6. The driving force generated in the focus direction causes the lens holder 2 to translate in the focus direction through the elastic supporting members 7.
A driving force in the tracking direction is generated by the electromagnetic driving circuit in which the tracking coils 4 are disposed in the gap in the magnetic circuit formed of the permanent magnets 5, the back yokes 6a and the opposing yokes 6b attached to the yoke base 6. The driving force generated in the tracking direction translates the lens holder 2 in the tracking direction through the elastic supporting members 7.
Next, the mechanism for suppressing torque around the Y axis which is a cause of inclination of the optical axis will be described referring to FIG. 24 to FIG. 26.
As shown in FIG. 24, displacement of the movable part composed of the objective lens 1, the lens holder 2, the focus coil 3, the tracking coils 4 and the printed boards 10 by dt in the X axis plus direction in the tracking direction causes the center of gravity, G, of the movable part to differ by the same displacement, dt, from the center of generation, Fc, of the focus driving force. Then the focus driving force Ff0 in the direction toward the optical disk E caused by the focus driving current If produces clockwise torque about the Y axis at the center of gravity, G, of the movable part.
On the other hand, as shown in FIG. 25, while the driving force Ft0 is produced in the tracking direction toward the X axis plus direction by the tracking driving current It in the sides of the tracking coils 4 that are parallel to the Z axis, driving forces Ft1 and Ft2 are produced in the opposite direction to the tracking driving force Ft0 in the sides of the tracking coils 4 that are parallel to the Y axis, for the magnetic flux passes therethrough in the focus direction. When the movable part is moved in the direction toward the optical disk E by the focus driving force Ff0, a difference occurs between the driving forces Ft1 and Ft2, which produces torque counterclockwise about the Y axis at the center of gravity, G, of the movable part.
That is to say, the torque around the Y axis produced by the focus coil 3 and the torque around the Y axis produced by the tracking coils 4 act in such directions as to cancel out, which suppresses inclination of the optical axis to allow the objective lens 1 to translate in the focus direction and in the tracking direction.
However, the conventional structure requires adjustment so that the torque around the Y axis generated by the focus coil 3 and the torque around the Y axis generated by the tracking coils 4 can accurately cancel each other, thus causing the problem that a large allowable error (hereinafter referred to as tolerance) can not be set for positional deviation of the magnets and positional deviation of the lens holder due to assembly error.
Furthermore, it requires that the focus coil and the tracking coils should be disposed in the same movable part. Therefore, in a separate type objective lens actuator in which the electromagnetic circuit, the movable part, and the supporting members for driving in the focus direction and the electromagnetic circuit, the movable part, and the supporting members for driving in the tracking direction are independently constructed, the torque due to the focus driving force can not be cancelled by the tracking coils, resulting in the problem that the optical axis of the objective lens may be inclined.